1. Field of the Invention
The present invention relates to an amplitude limiter circuit for generating a signal including continuous oscillation and gradually damping oscillation following said continuous oscillation, for use as an erasing signal or the like in a magnetic recording and/or reproducing apparatus and, particularly, it relates to means for enabling reducing the damped oscillation finally to zero.
2. Description of the Related Art
In a magnetic recording and/or reproducing apparatus for recording video signals at a period of one field or one frame onto a recording medium, such as a magnetic disc, in case where a recording track 2 is formed concentrically on a magnetic disc 1 as shown in FIG. 2, for example, it is desirable that each track as formed on the disc can be independently erased. In such case, it is preferably to use an erasing signal including continuous oscillation T2 having a frequency out of the the frequency range of a video signal to be recorded and gradually damped oscillation T3 following said continuous oscillation (see FIG. 3A) since such an earsing signal enables a magnetic head to operate within a high permeability range, so that each recording track as recorded on the recording medium can be erased in an efficient manner (see Japanese Laid-Open Patent Application No. Sho 84-110008).
Heretofore, a circuit as shown in FIG. 4, for example, has been used to generate a signal including continuous oscillation and damped oscillation following said continuous oscillation. The circuit as shown in FIG. 4 includes an oscillator 3 for generating continuous oscillation as shown in FIG. 3B, an amplitude modulator 4, a trapezoidal wave generating circuit 5, and a control circuit 7. The trapezoidal wave generating circuit receives a pulse as shown in FIG. 3C from the control circuit 7 and generates a trapezoidal wave as shown in FIG. 3D. The amplitude modulator 4 effects amplitude modulation of the continuous oscillation fed from the oscillator 3 by the trapezoidal wave fed from the trapezoidal wave generating circuit 5 and feeds the modulated signal to an output terminal 9. The output signal at the terminal 9 is passed through a filter 10, which removes unnecessary higher harmonic waves, and is then subjected to linear amplification by an amplifier 11. The amplified signal is recorded by a magnetic head 12 onto the recording medium 1, as an erasing signal, to erase a video signal thus far recorded on the recording medium. For example, during the continuous osciallation period T2 as shown in FIG. 3A, the magnetization in the recording medium 1 is subjected to saturated magnetization by the erasing signal, so that the original signal previously recorded on the recording medium is erased by the superimposed erasing signal, and then distortion is removed during the damped oscillation period T3. The periods T2 and T3 are so determined that the above conditions are satisfied.
However, the conventional device as shown in FIG. 4 has a drawback as described below. That is, this device requires delicate adjustment of the potential of the trapezoidal wave fed to the amplitude modulator 4, and if this potential is subjected to a drift owing to temperature change, a carrier (continuous oscillation) having amplitude shown by a in FIG. 5 would remain in the erasing signal and it might be impossible to make efficient erasing oscillation.